Generally, a refrigerant circuit is constructed of a compressor for compressing a refrigerant, a radiator for cooling the refrigerant, an expansion valve for expanding the refrigerant, and an evaporator for heating the refrigerant, which are connected serially. In the refrigeration cycle of this refrigerant circuit, the refrigerant undergoes a pressure drop at the expansion valve from a high pressure to a low pressure while expanding, at which time the internal energy is released. Accordingly, when the pressure difference is large between the low pressure side (the evaporator side) and the high pressure side (the radiator side) in the refrigerant circuit, the released internal energy becomes relatively great. As a consequence, the energy efficiency of the refrigeration cycle degrades considerably.
In view of such a problem, various techniques have been proposed for recovering the internal energy of the refrigerant that is released at the time of the expansion. For example, JP 2006-266171 A and Document 1 (International Refrigeration and Air Conditioning Conference at Purdue, Jul. 17-20, 2006, R169, “BASIC OPERATING CHARACTERISTICS OF CO2 REFRIGERATION CYCLES WITH EXPANDER-COMPRESSOR UNIT”) propose refrigeration cycle apparatuses that perform power recovery using, as a power recovery mechanism, a positive displacement-type fluid machine in which an expansion mechanism and a blower (sub compression mechanism) are coupled to each other by a shaft.
In the refrigeration cycle apparatus as disclosed in JP 2006-266171 A and Document 1, the positive displacement-type fluid machine and a main compression mechanism are accommodated in separate closed casings respectively, as shown in FIG. 6 of JP 2006-266171 A. An oil reservoir for holding refrigeration oil that is supplied for the positive displacement-type fluid machine and the main compression mechanism is provided in each of the closed casings.
However, the amount of the refrigeration oil discharged from the main compression mechanism to the refrigerant circuit is not always equal to the amount of the refrigeration oil discharged from the positive displacement-type fluid machine to the refrigerant circuit. Normally, one of the amount of the refrigeration oil discharged from the main compression mechanism to the refrigerant circuit and the amount of the refrigeration oil discharged from the positive displacement-type fluid machine to the refrigerant circuit is greater than the other. Consequently, the amount of the refrigeration oil held in one of the oil reservoirs and that held in the other oil reservoir may be off-balanced when separate oil reservoirs are provided respectively for the closed casing accommodating the positive displacement-type fluid machine and the closed casing accommodating the main compression mechanism as in the refrigeration cycle apparatus disclosed in JP 2006-266171 A and Document 1. In other words, one of the closed casings may contain an excessive amount of refrigeration oil while the other closed casing may contain too little refrigeration oil. In that case, lubrication and sealing of the positive displacement-type fluid machine and the main compression mechanism may not be performed appropriately.
For example, Document 1 discloses that an oil separator is disposed between the main compression mechanism and a radiator so that the refrigeration oil recovered by the oil separator is supplied to the closed casing that accommodates an expansion mechanism-sub compression mechanism unit. This inhibits, for example, a decrease in the amount of the refrigeration oil held in the closed casing that accommodates the expansion mechanism-sub compression mechanism unit.
Nevertheless, even when the oil separator is provided between the main compression mechanism and the radiator as described in Document 1, it is still difficult to suppress a decrease in the amount of the refrigeration oil held in the oil reservoir within the closed casing that accommodates the expansion mechanism-sub compression mechanism unit or in the oil reservoir within the closed casing that accommodates the main compression mechanism sufficiently. The reason is that, even when the refrigeration oil recovered by the oil separator is supplied to the closed casing that accommodates the expansion mechanism-sub compression mechanism unit, the amount of the refrigeration oil held in the oil reservoir within the closed casing that accommodates the expansion mechanism-sub compression mechanism unit decreases when the amount of the refrigeration oil discharged from the expansion mechanism-sub compression mechanism unit to the refrigerant circuit exceeds the amount of the refrigeration oil recovered by the oil separator. Also, the amount of the refrigeration oil held in the oil reservoir within the closed casing that accommodates the main compression mechanism decreases when the amount of the refrigeration oil discharged from the main compression mechanism to the refrigerant circuit is relatively large.